The present invention relates to transmissive liquid crystal display devices used for office automation (OA) equipment such as word processors and personal computers, mobile information terminals such as personal digital assistants and mobile telephones, or camcorders equipped with a liquid crystal monitor.
A liquid crystal display device usually comprises a liquid crystal cell, a polarizer and an optical compensation sheet (retardation plate). A transmissive liquid crystal display device comprises a pair of polarizers, a liquid crystal cell sandwiched therebetween, a single or plurality of optical compensation sheets disposed between the liquid crystal cell and either one or both of the polarizers.
The liquid crystal cell comprises rod-like liquid crystalline molecules, a pair of substrate for enclosing the molecules and electrode layers for applying an electric voltage to the molecules. Examples of the mode of a liquid crystal cell include TN (Twisted Nematic), STN (Super Twisted Nematic), ECB (Electrically Controlled Birefringence), IPS (In-Plane Switching), VA (Vertical Alignment), OCB (Optically Compensated Birefringence), HAN (Hybrid-Aligned Nematic), ASM (Axially Symmetric Aligned Microcell), Half Tone Gray Scale modes, domain divided mode, and display modes using a ferroelectric liquid crystal and an antiferroelectric liquid crystal.
The transmissive liquid crystal display devices can not avoid problems concerning viewing angle such as reduced display contrast, changes in display color and reversed gradation occurring when viewed obliquely because of the refractive index anisotropy of the liquid crystal molecules and thus has been demanded to be improved in these regards.
For a transmissive liquid crystal display device using a TN mode (twisted angle of liquid crystal is 90 degrees), a method for solving these problems has been proposed and practically used wherein optical compensation films are disposed between the liquid crystal cell and each of the upper and lower polarizers.
For example, there are known some structures wherein an optical compensation film composed of hybrid-aligned discotic liquid crystal or nematic hybrid-aligned liquid crystalline polymer is disposed between the liquid crystal cell and each of the upper and lower polarizers (Patent Document Nos. 1 to 3 below).
However, the TN mode can improve the range where contrast widens but is wide in the range where gradation reverses and thus is not necessarily sufficient in viewing angle characteristics. This is because the range where gradation reverses is widened by the molecules in the liquid crystal cell slanting up to the 90 degree direction due to that the liquid crystal layer is twisted at 90 degrees.
For the reasons described above, the mode of a liquid crystal cell is preferably a display mode using an ECB mode wherein the liquid crystal molecules are twisted at an angle of zero degree and homogeneously aligned in the sense of narrowing the range where gradation reverses. An arrangement for improving the viewing angle of the ECB mode is proposed wherein two nematic hybrid-aligned optical compensation films and two uniaxial retardation films are disposed so that each of the compensation films and each of the retardation films are located above and below the homogeneous liquid crystal cell, respectively (Patent Document No. 4).
However, this method can not solve the problems concerning viewing angle such as reduced display contrast, changes in display color and reversed gradation occurring when the liquid crystal display device is viewed obliquely and has left problems including large fluctuations in displaying characteristics due to the variation in the parameter of each film, the increased total film thickness, and the lower reliability of the device because of the use of four film in total above and below the cell, all of which have been demanded to improve.                (1) Patent Document No. 1: Japanese Patent Publication No. 2640083        (2) Patent Document No. 2: Japanese Laid-Open Publication No. 11-194325        (3) Patent Document No. 3: Japanese Laid-Open Publication No. 11-194371        (4) Patent Document No. 4: Japanese Laid-Open Publication No. 2005-202101        